1. Field of the Invention
This invention relates to the removal of dissolved metals from water, and more particularly concerns polymers and the use thereof for the absorptive removal from aqueous media of trace amounts of dissolved ionized species of metals.
2. Description of the Prior Art
Various techniques are known for removing dissolved ionized metals from water. Activated carbon has long been used for the absorption of dissolved metals from aqueous media. The most significant use of activated carbons for absorption of dissolved metals is in gold mining operations. In such use, activated carbon granules absorb gold in the form of aurocyanide anion produced by the leaching of gold ore with dilute solutions of sodium cyanide. Although effective in said removal of gold from ore, long contact times on the order of 24 hours are required in a tumbling operation.
The removal of nuisance or toxic metals from aqueous streams has generally been accomplished by precipitative methods wherein an ingredient capable of forming an insoluble compound with the dissolved metal is added to the stream. Sufficient interaction time is usually achieved in a holding vessel wherein the insoluble compound forms and grows into a crystalline solid of filterable size. The solid is then removed by settling, filtration or centrifugation, and is usually discarded as a sludge which in fact may require expensive disposal costs.
The separation of dissolved species from water may also be achieved by way of reverse osmosis techniques wherein a pressurized aqueous stream is fed to a selectively permeable membrane. Although reverse osmosis operations are successful in specialized applications, the requisite high pressurization is costly, and the membranes are susceptible to fouling. Also, a concentrated rejectate stream containing the removed species may constitute a significant disposal problem.
Ion exchange resins of strong acid or strong base functionality have been used for the absorption of commonly abundant dissolved cations and anions, respectively. However, commonplace ion exchange resins cannot remove trace ions because they indiscriminately absorb commonly abundant innocuous ions such as Na, K, Mg, Al, Ca, etc., leaving no residual absorption capacity for trace species. Specialized ion exchange resins, referred to as “chelation” resins are known which selectively absorb only “heavy” metals. Such specialized resins are usually comprised of beads of styrene/divinylbenzene polymer having grafted iminodiacetic acid groups. “Heavy” metals are generally toxic species, usually found in only trace levels in natural waters or industrial effluents. The heavy metals may be further characterized as transition group metals classified in groups I B through VIII B of the Periodic Table, and generally characterized in having incomplete inner rings of electrons or being otherwise capable of existing in more than one valence state.
When a metal-saturated chelation resin must ultimately be disposed of, it is found that the preferred method of disposal, namely incineration, is not a viable option because of the large amounts of volatile aromatic hydrocarbons generated during incineration. The aromatic hydrocarbons are considered to be toxic substances. Although said aromatic hydrocarbons can be removed from the incineration gases by activated carbon, large amounts of the carbon are required.
Chelation resins are not generally capable of removing toxic species to extremely low concentration. For example, where it is desired to remove species such as mercury or lead to concentrations below one part per billion (ppb), chelation resins are usually ineffective. One explanation for said shortcoming is that forces of bonding that hold the metal ion are not sufficiently strong to prevent some dissociation.
The use of aliphatic polymers of nitrilotriacetic acid (NTA) and polyethylenimine (PEI) for the selective absorption of dissolved ions has been disclosed in U.S. Pat. Nos. 3,715,339; 4,332,916; 5,002,984; 5,597,850; and elsewhere. Said PEI-based polymers, having recurring amine groups and iminodiacetic acid groups, absorb cationic species by two different mechanisms. In one mode of function, the amine groups serve as ligands which form a metallo-organic coordination compound with the cation. In a second mode of function, the iminodiacetic acid groups form a chelation ring which includes the absorbed cation. Whereas some cations, such as a copper, cadmium and lead become strongly bound to the polymer, presumably because of bonding via both mechanisms, other heavy metal cations are not sufficiently bound to produce residual concentrations below 1 ppb.
U.S. Pat. No. 6,521,340 discloses the vapor phase reaction of carbon disulfide with water-swollen particles of NTA/PEI polymer in a sealed container to incorporate dithiocarbamate groups into said polymers. The effect of the incorporation of said dithiocarbamate groups is to cause the polymer to exhibit higher affinity for cationic heavy metal ions. In particular, aqueous streams treated with the CS2-modified polymer show smaller effluent concentrations of heavy metal cations.
It has been found, however, that said dithiocarbamate-containing polymers have poor storage stability. In the course of storage in sealed industrial containers at temperatures in the commonplace range of 40° F. to 80° F., the dithiocarbamate groups split off the polymer with evolution of H2S. Not only does such decomposition downgrade the capabilities of the polymer, but the H2S which accumulates within the container is hazardous because of its toxicity and flammability.
It is accordingly an object of the present invention to provide a process for the selective removal of heavy metals from a flowing stream of water.
It is a further object of this invention to provide a process as in the foregoing object which employs a stationary bed of polymer in particulate form, through which said stream is caused to flow.
It is another object of the present invention to provide a process of the aforesaid nature wherein sulfur-based functional groups are incorporated into said polymer during passage of said stream through said bed.
It is yet another object of this invention to provide a process of the aforesaid nature wherein said metals are in dissolved ionic form, and said heavy metals are accompanied by larger amounts of non-heavy metals.
These objects and other objects and advantages of the invention will be apparent from the following description.